Bi-Directional Equalization System for Li-Ion Battery Pack Based on Fly-back Transformer

For balancing Li-ion battery cells connected in series and effectively improving the consistency of the cells, a bi-directional equalization system based on fly-back transformer is proposed. Unlike the passive equalization technology using a resistor or active equalization with expensive DC-DC converter for the balancing among the cells, this equalization circuit consists of the fly-back transformer and RCD circuit, which can easily and cheaply realize the energy transfer between the whole battery module and the cells, and thus achieving bidirectional equalization. In this system, both the primary side and the secondary side of multi-winding transformer are connected to a MOSFET. All MOSFETs are controlled by the PWM signal. The control timing and duty ratio of the PWM control signal are determined through the simulation analysis. Meanwhile, an RCD circuit is applied at the primary side of multi-winding transformer for buffering the peak voltage caused by leakage inductance. In the control process, the equalization strategy is proposed by considering the voltage, SOC and capacity difference simultaneously, and in order to avoid the equalization control logic chaos, only one battery cell is balanced at the same time. The equalization strategy is verified by MATLAB/Simulink and has a satisfactory performance. To verify the effectivity of the proposed circuit, a prototype for six series-connected 18650 NCM lithium battery cells is implemented, and the experimental results show that the equalization system has good circuit performance and the efficiency is more than 80%. This proposed method also has potential to balance the state of health of cells in the long term.